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Creating Living Hinges 



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Creating Living Hinges 

Written By: Kevin Gunn 

SUMMARY 

I have a particular interest in discovering new ways to push the limits of material fabrication 
with my laser cutter. In this guide I'm going to give you an overview of living hinges that will 
allow you to make bends (and springs!) in a single piece of rigid material. Wood and acrylic 
are particularly amenable to this technique, and the result has an artistic aspect that's pretty 
cool as well! 

More importantly, I'm going to try to add a bit of theory to this so that you can extend these 
techniques in new and better ways. 



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Creating Living Hinges 



Step 1 — Creating Living Hinges 




• What does it mean for a material to be rigid in the sense I'm using the term? A rigid 
material is one that has very limited ability to bend over a given length of that material 

• Acrylic is RIGID. Very, very little flexibility, but it's a beautiful material. So how do we get it 
to bend? 

• The acrylic sample that I cut here has the ability to fold easily from fully open (flat) all the 
way to fully closed (180 bend) in either direction. It also can act as a spring along its 
length. 

• How does this work? Head to the next step for the physics and geometry of it. 



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Step 2 



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• I use Google Sketchup for my 
design work. Here's a screen 
capture of the simplest method for 
making living hinges: a zigzag. 

• Remember how I defined "rigid" in 
Step 1 ? Acrylic will only bend 
naturally VERY slightly per unit 
length before it breaks (wear safety 
glasses if you test this). 

• Here the acrylic sample is 120mm 
long, 40mm wide and 5.5 mm thick. 
Maximum possible bend over the 
length is just a couple or three 
degrees without the living hinge. 

• What I'm doing here is effectively 
EXTENDING THE LENGTH of the 
acrylic to make a big bend in a 
small space. The "bend zone" in 
the image is only 39mm, but the 
zigzag means the bend is occurring 
over about 9 zigs of 40mm each 
(360 mm). In effect, the hinge is 3 
times the length of the total piece. 

• Additionally, the 5.5 mm acrylic is 
now THINNER in the hinge zone 
since the zigzags are only 2mm 
wide. This also increases the 
amount of bend that is possible. 



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Creating Living Hinges 



Step 3 



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• The problem with this design is that 
it is FRAGILE. Notice that 
throughout the hinge area that the 
amount of acrylic connecting the 
zigzags to each other is only 2mm 
thick. 

• Also, that 2mm of acrylic only 
exists at the top or the bottom of 
each zigzag -- that's about as 
fragile a place as they could 
possibly be! 

• Finally, this design (while VERY 
flexible and easy to understand) 
allows a lot of rotation in directions 
outside of the hinge axis (i.e. you 
can twist it easily). 

• So... can we improve on this 
design now that we understand why 
it works? Of COURSE we can! 



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Step 4 



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• By essentially overlaying two 
zigzags in our design, we can 
maintain a high degree of flexibility 
in our hinge. 

• The extra connections this creates 
within the material greatly enhance 
the strength of the object. 

• This also reduces the ease with 
which the material can twist and 
stretch (and that may or may not 
be a good thing depending on your 
goals). 

• PROBLEM: One out of two zigs 
have TWO connections, but the 
other half have ONE. That means 
we STILL have weak spots. 

• SOLUTION: See how a pattern of 
offset gaps is emerging? Let's take 
that one step further with a "Triple 
Zig" (next step). 



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Step 5 



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Creating Living Hinges 




• Finally, this staggered layout which 
I call a "Triple Zig" is the strongest 
variant I've tried so far. I've seen 
other folks using similar cuts as 
well so I'm probably not the first to 
do it this way, though I haven't see 
others doing exactly this cut in 
acrylic yet. Acrylic is a bit less 
forgiving than wood, so I 've settled 
on the 2mm gap / 2mm zig width as 
working best with 5.5 mm (1/4- 
inch) acrylic. 

• Notice that each "zig" has TWO 
connections to its neighbors. This 
almost eliminates twist. 

• @solarbotics has had some luck 
with slits in acrylic and @9600 and 
@talldarknweirdo have been 
working in this area as well. I think 
we were all inspired by the work in 
wood done by snijlab here: 
http://www.snijlab.nl/?page_id=358 

• This pattern has good strength, 
good flexibility and low twist. It also 
looks great! I'm hoping to create 
some electronics enclosures with 
this technique in the near future. 

• I hope you found this technique and 
its explanation helpful. Please 
explore and build on it and share 
back what you learn with the Maker 
Community. Open Sourcing 
knowledge makes the whole world 
richer. Thanks! 



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Creating Living Hinges 

This document was last generated on 201 2-1 0-31 1 1 :31 :48 PM. 



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